Light-dependent modulation of hydrogen peroxide response by phytochrome-related photosensor RcaE in Fremyella...

This study investigates the differential responses of wild-type (WT) and ΔrcaE (phytochrome-related photosensor RcaE deficient) strains of the cyanobacterium Fremyella diplosiphon, a cyanobacterium capable of chromatic acclimation, to hydrogen peroxide (H₂O₂) under green light (GL) and red light (RL). Our findings reveal that light quality significantly modulates H₂O₂ sensitivity, with RcaE playing a central role. The ΔrcaE strain displayed higher basal reactive oxygen species (ROS) but maintained photochemical efficiency better than WT under RL. The stability of phycobiliproteins and other photosynthetic pigments was differentially impacted in both strains under varying light and H₂O₂ concentrations. Morphological responses also differed, with the ΔrcaE strain displaying a more resilient spherical shape under RL. Morphological analysis revealed that light quality and RcaE functionality influence filament fragmentation and structural integrity under oxidative stress. The WT strain exhibited higher sensitivity to H₂O₂ under GL, correlating with enhanced catalase and ascorbate peroxidase activities. Conversely, the ΔrcaE strain showed increased superoxide dismutase and glutathione reductase activities under RL, indicating a greater reliance on superoxide radical scavenging. The oxidative stress markers, such as ROS, malondialdehyde (MDA) and H₂O₂ levels, were differentially impacted in both strains. These results highlight distinct light-dependent and RcaE-mediated acclimation strategies to oxidative stress in F. diplosiphon. The WT strain prioritizes direct H₂O₂ detoxification under GL, while ΔrcaE relies more on superoxide radical management, particularly under RL. This study contributes to understanding how light and RcaE photoreceptor influence cyanobacterial resilience to oxidative stress, crucial for their ecological success in fluctuating light environments in aquatic ecosystems.